1. Technical Field
The present invention relates to video signal processing in general, and in particular, to the processing of ordered dither images for displaying in an interlaced field format.
2. Description of the Prior Art
Video signals are invariably generated in an analog form. To obtain the advantages of digital transmission, it is necessary to quantize the signal in some way. In ordinary quantization, the output levels of a quantizer are uniformly spaced throughout the range of the input signal. For example, in the absence of any coding, it would require six bits to send a signal quantized to 64 levels. And, in practice, at least 64 levels are required to produce a high quality picture.
A strong incentive to reduce the number of levels exists since it would reduce the number of bits that has to be transmitted. If the quantizer step size is doubled, for example, the number of levels can be halved; and the bit rate of the source can be reduced from, say, six to five bits per sample. If this is done, the picture quality is degraded, but primarily for only one type of picture material, i.e., those areas in which the luminescence changes slowly or low-detail areas. The degradation takes the form of curved lines which look very much like contour lines on a map; thus, this type of degradation is referred to as contouring. In order to eliminate the objectionable effect of contouring, an effect similar to increasing the number of levels can be achieved by adding a dither signal to the input signal. This deterministic or ordered dither was originally introduced by J. O. Limb in an article entitled "Design of Dither Wave Forms for Quantized Visual Signals," Bell System Technical Journal, Vol. 48, pp. 2555-2582 (1969).
Ordered dither, or more simply, dither, is a video signal processing technique which provides bi-level display systems with gray-scale, or continuous-tone capability. Images in bi-level display systems typically comprise a matrix of individually closely spaced display cells, each of which resides in one of two visual states. That is, each display cell is either completely energized ON or completely deenergized OFF.
Dither processing is one known technique for determining which cells should be ON and which should be OFF. The image to be produced is divided into a matrix of picture elements (PELs), each PEL corresponding to a respective cell in the display system. A dither threshold value is assigned to each PEL from a predetermined dither matrix. If the intensity of any given PEL is greater than the dither threshold value assigned thereto, the corresponding display cell is turned ON. Otherwise, it is maintained OFF.
Although ordered dither was developed principally for use in bi-level display systems, in some instances, it is desirable to display dithered images on a conventional cathode ray tube (CRT) display using the CRT in a bi-level mode. Such arrangements advantageously minimize both transmission bandwidth requirements and display hardware costs at the present time. In addition, CRT displays are presently less expensive than plasma or other bi-level display panels.
A problem arises, however, because conventional CRT systems use an interlaced field format. When a dithered image is displayed in this format, the two fields of each frame have unequal brightness because of the way in which the threshold values of a dither matrix are spatially distributed therein. The result is an annoying flicker in the displayed image. One technique for minimizing this flicker was proposed in U.S. Pat. No. 3,953,668, issued to C. N. Judice on Apr. 27, 1976. In this arrangement, the flicker, which results when an ordered dither image is displayed in an interlaced field format, is eliminated by energizing each PEL of te display to have an intensity proportional to the average intensity of a cluster of PELs of the dithered image, rather than the intensity of a single one of the PELs. While such an arrangement has been found satisfactory in a bi-level display system in which the receiver incorporates a circuit for displaying the average intensity of a cluster of PELs, some arrangements do not readily allow for the receiver modification required for this approach.